1. Field of the Patent Application
The present disclosure is generally directed to contact assemblies for use with an electric power switching apparatus. More specifically, the present disclosure is generally directed to a moving contact assembly and a stationary contact assembly for use with an electric power switching apparatus. Such moving and stationary contact assemblies allow the switching apparatus to achieve a high withstand and close-on current ratings. For example, such withstand and close-on current ratings may achieve withstand and close-on current ratings of more than 10,000 A. In one arrangement, the present disclosure relates generally to a contact assembly arrangement that may be used for electrical power transfer and, more particularly, may be used in an automatic transfer switch. However, aspects of the invention may be equally applicable in other scenarios as well.
2. Description of Related Art
An automatic transfer switch is designed to provide a continuous source of power for critical loads by automatically transferring from a normal power source to an emergency power source when the normal power source falls below a preset limit. Automatic transfer switches are in widespread use in airports, subways, schools, hospitals, military installations, industrial sites, and commercial buildings equipped with secondary power sources and where even brief power interruptions can be costly or perhaps even life threatening.
Transfer switches operate, for example, to transfer a power consuming load from a circuit with a normal power supply to a circuit with an auxiliary power supply. A transfer switch can control electrical connection of utility power lines and the diesel generator to facility load buses. In certain installations, the transfer switch automatically starts a standby generator and connects the standby generator to the load bus upon loss of utility power. In addition, the transfer switch can automatically reconnect the utility power to the load bus if utility power is reestablished.
Automatic transfer switches are typically multi-pole switches. Therefore, an automatic transfer switch used with a three phase, four wire system will typically include three poles for switching the three phase conductors of the load between the three phase conductors of the normal power source and the three phase conductors of the emergency power source. The fourth, neutral conductor of the load is often permanently connected to the neutral conductors of the normal and emergency sources.
Unlike circuit breakers, which are designed to promptly open contacts (see for example, U.S. Pat. No. 6,977,568 which is herein entirely incorporated by reference and to which the reader is directed for further information), transfer switches are designed to remain in a closed state during certain fault conditions. For example, certain known circuit breakers employ the electromagnetic forces to blow contacts open and limit the amount of fault current that a system downstream may see. Certain transfer switches utilize contacts similar to circuit breaker design and overcome the blow off contact forces by employing mechanical operators with toggles.
Therefore, when used in a distribution system with circuit breakers, the transfer switch must have sufficient short circuit fault capability and protection so as to withstand and/or close-on short circuit and stay closed long enough to allow circuit breakers to open under the appropriate circumstances.
Accordingly, it would be desirable to provide a cost-effective automatic transfer switch that is easy to assemble and install. There is also a general need for an automatic transfer switch that can provide a sufficient short circuit fault protection while also allowing the contact assembly configuration to stay closed long enough to allow a system circuit breaker to open under the appropriate circumstances. There is also a general need for an enhanced automatic transfer switch designed to withstand and/or close on very high short circuit fault currents by employing electromagnetic forces to keep contacts closed.